Find the cost of 80 Orange at 5 for 30 naria

By selling 80 oranges a man gains a cost price of 20 oranges. What is the gain percentage?
To solve this tricky question, let the cost price (CP) of an orange is 1 Rs.

Now CP of 80 oranges = 80 Rs………..(1)

And we know that

Profit= SP (selling price) - CP (cost price)

Here;

Profit = SP of 80 oranges - CP of 80 oranges…….(2)

{Here profit is already given that is CP of 20 oranges }

By (2);

CP of 20 oranges = SP of 80 oranges - CP of 80 oranges

Or

CP of 100 oranges = SP of 80 oranges

Or

SP of 80 oranges = 100 Rs…………(3)

{ cp of 1 orange is 1 Rs.}

Here we have

CP = 80 rs.

SP = 100 rs.

% profit = (SP - CP)*100/CP

% profit = (100–80)*100/80 = 20*100/80 = 25%

So %profit is 25%